what is the threat model that they protect against and are vulnerable to?
Hardware Wallets are security appliances with varying hardware and features, but the overall concept is largely identical. A trusted device holds cryptographic keys, allows for viewing of information on a dedicated display, and accepts secure input through its own interface (buttons ...
See here the latest list of 27 hardware wallets, many use open source software in the meantime - flagged with a (O). The other ones are flagged with a (N).
For security reasons it is of course not sufficient to consider the software alone, also hardware can contain backdoors.
(O) Archos Safe-T (Mini)
(O) BC Vault
(O) BitLox Hardware ...
There is a standard algorithm for generating the keys from the seed words (BIP 39 / BIP 32). Most hardware wallets follow this algorithm (check before you buy). So even if the hardware device has failed or cannot be used, you can still use your written seed words with any other software or hardware device that implements the standard algorithm to ...